Combine all stock ingredients in a large pot with 1 tbsp kosher salt (reserve the 2 tbsp chopped fresh dill for later). If you're salt-sensitive, you can start with 2 tsp salt, however, please note that for the saffron flavor to shine here, you do need salt in the mix. I really recommend 1 tbsp if you're not watching your salt intake.
Cover ingredients with 4 quarts (16 cups) water. Bring to a boil.
Reduce heat to a simmer (around medium heat). Let the stock cook uncovered for 50 minutes until the liquid reduces by about a third and the stock is flavorful.
While the stock is cooking, make the matzo balls. Whisk eggs in a small mixing bowl with vegetable oil and seltzer water. Use a fork to stir in the matzo meal, baking powder, salt, garlic powder, onion powder, and chopped fresh dill to make a thick batter. Place the batter in the refrigerator for 30 minutes.
While the matzo balls are chilling, slice 2 carrots, 3 celery stalks and 2 zucchinis
While the batter is chilling, in a second pot combine 3 quarts (12 cups) of water with a pinch of saffron and 1 tbsp salt. Bring to a simmer, stir to dissolve the salt. Keep the salted saffron water hot until the matzo ball mixture is fully chilled.
Place a small dish of cold water beside the pot of saffron water. Take the chilled matzo ball batter out of the refrigerator. Bring the saffron water to a simmer. Wet your hands and roll the matzo ball batter into walnut-sized balls, then gently drop them into the simmering saffron water. Wet your hands between each ball; this makes forming them easier. Don't make them larger than walnuts, they'll expand a lot as they cook!
When all of the matzo balls are in the water, bring back to a low bubbling simmer (not a heavy boil) and cover the pot. Let the matzo balls simmer for 30-35 minutes, keeping the pot covered for at least the first 30 minutes until it's cooked through. No peeking! Keeping the pot covered will help the matzo balls become fluffy.
When the vegetable stock liquid has reduced by about a third, strain the stock through a fine-mesh strainer or cheesecloth into a large bowl.
Rinse the pot and pour the stock back into the pot. Add the fresh sliced carrots, celery and zucchini to the stock along with 1 tbsp chopped dill. Adjust seasoning to taste, adding more salt if desired. Cook for another 10-15 minutes until the vegtables are tender.
A note about the matzo balls: this recipe contains baking powder. If you keep kosher, there are, in fact, several brands of kosher for Passover baking powder (Gefen makes a popular one sold in kosher markets). It makes for lighter, fluffier matzo balls. If you're not concerned about fluffiness to feel free to omit; your matzo balls will turn out more dense, like "sinkers."
A note about saffron: Invest in good quality saffron. I say invest because saffron can be very pricey. If a bottle of saffron threads costs less than $10, it’s probably not worth buying. Imitation saffron often sold in a bag under the name "American Saffron" or "Mexican saffron," do not impart the flavor of true saffron-- in fact, they have no flavor at all and are used mostly to color foods. I don’t recommend the bag full of saffron that costs $5… it’s not the stuff you want, and it won’t give you the flavor you need.